So, let's say I go buy an 360 Slim today. I can use my 120 gig hard drive in in, right? I just have to take it out of the casing? I don't want to lose all my sweet 2K13, Batman, and WWE 13 data.
I used a Torx screwdriver set to take my hard drive out of my old 20-gig model and put it into my new 4-gig one. It was actually really easy, and there are lots of how-to videos online.
You won't have any problem, I'm guessing. But you do need those screwdrivers if you don't have them.
EDIT: Here's a pretty simple video tutorial.